1. Field of the Invention
The present invention relates to an electro-mechanical-acoustic transducing device which is used for generating a ring-back calling tone or vibration in portable terminal devices such as a portable telephone, a pager, a personal handy phone set, and the like.
2. Description of the Related Art
As means for informing a user that a call is coming, a conventional portable terminal device (e.g., a conventional portable telephone, pager, personal handy phone set, or the like) generally includes both a small beeper for generating a bell sound and a micromotor in which vibration is caused by a weight eccentrically attached to a rotation shaft. On the other hand, U.S. Pat. No. 5,524,061 discloses a portable telephone containing an electro-mechanical-acoustic transducing device which realizes the generation of sound or vibration for informing a user that a call is coming using one unit in order to reduce the size and weight of a portable terminal device.
According to the electro-mechanical-acoustic transducing device disclosed in above-mentioned U.S. Pat. No. 5,524,061, by supplying an electric signal to an electromagnetic coil of an electromagnetic driver which is attached to a soundboard, a magnetic motional mass unit is vibrated due to a magnetic force affecting a region between the electromagnetic coil and the magnetic motional mass unit. The vibration is transferred to the soundboard, thereby generating vibration and sound. Such vibration becomes intensified in the vicinity of a resonance frequency of a mechanical resonance system constituted by the magnetic motional mass unit and a non-linear spring that supports the magnetic motional mass unit. Thus, the electric signal to be supplied to the electromagnetic coil is set to have a predetermined range of frequency containing the aforementioned resonance frequency (more accurately, so as to repeatedly sweep the predetermined frequency range in a successive manner) by a sweep frequency generator. The vibration reaches its maximum when the mechanical resonance frequency of the magnetic motional mass unit and the non-linear spring coincides with the frequency of the electric signal supplied to the electromagnetic coil.
According to such a conventional system, however, the supplied electric signal is swept in the entire frequency range including frequencies which do not correspond to the mechanical resonance frequency. The vibration becomes small when the electric frequency does not correspond to the mechanical resonance frequency upon sweeping. Therefore, in the above conventional system, an efficiency in the output of mechanical vibration with respect to an electrical input is often low.